As the cost of primary energy sources, such as oil, natural gas and electricity, continue to increase, many consumers are turning to wood, coal, etc., burned slowly in a stove as a source of heat. Many such stoves use a mechanical thermostat to control air intake for combustion of the fuel.
It is highly desirable to have an operating temperature in a solid fuel stove (for descriptive purposes only the term "solid fuel" will be restricted to "wood" hereinafter), which remains very constant during burning time. Moreover, the ideal temperature of the stove is neither too high nor too low. Large oscillations of stove temperature will, of course, result in rapid, uncomfortable variations of room temperature. If the stove temperature is too high, there will also be discomfort, plus excessive consumption of wood and a greater risk of fire in the stove pipe or chimney. On the other hand, if the stove temperature is too low, it will result in discomfort, poor efficiency, excessive build-up of creosote or even extinguishment of the fire.
Given the above requirements of temperature control in a wood-burning stove, it is clear that the regulating means for the temperature must possess certain characteristics. Clearly, the most important characteristic is that the regulating means must be sensitively and quickly responsive to change in the operating temperature of the stove within a given temperature range.
In wood-burning stoves of the above type, the regulating means usually consists of a thermostat linked to an air intake arrangement. The thermostat is adapted to sense the temperature of the stove, and then by the intermediary of a linkage, modulate the effective open area of the air intake.
For example, the Canadian Patent No. 1,157,718 issued to Stockton Barnett on Nov. 29, 1983 teaches the use of a thermostat which moves a damper arrangement to open and close the air intake according to fluctuations in temperature of the stove. The relationship between the variation of the air intake area and temperature variation is expressed by the mathematical formula .DELTA.A=K.DELTA.t.sup.2 wherein .DELTA.A is the variation of the air intake area; K is a constant, and .DELTA.t is the variation in the operating temperature of the formula.
This formula is generally adequate for stove wall temperatures in the range of 200.degree.-300.degree. F. However, in regions where winter outdoor temperatures can become very cold, it is necessary for proper heating of a dwelling to have stove wall temperatures often greater than 350.degree. F. This is a first shortcoming of the cited Patent.
Moreover, if the stove temperature rises to become too hot, there will result, according to the above equation, a rapid closure of the air intake. But the stove temperature will continue to rise for a certain period of time and, with the air intake still partially or fully closed, this will result in snuffing the combustion of the wood or even extinction of the fire. Assuming that the fire is not completely extinguished, there will follow a significant temperature drop causing a rapid opening of the air intake and the start of another similar cycle. This constitutes a second shortcoming of the cited Patent.